Nuclear Physics B (Proc. Suppl.) 196 (2009) 153–158
www.elsevier.com/locate/npbps

The current status of the GRAPES-3 extensive air shower experiment
S.K. Guptaa , H.M. Antia a , S.R. Dugada , U.D. Goswamia , Y. Hayashib, A. Iyera , N. Ito b , P.
Jagadeesana, A. Jaina , S. Karthikeyana∗ , S. Kawakamib, M. Minaminob , P.K. Mohantya , S.D. Morrisa,
P.K. Nayaka , T. Nonaka b , A. Oshimaa , B.S. Raoa , K.C. Ravindrana , H. Tanakaa , S.C. Tonwara(The GRAPES-3 Collaboration)ab

Tata Institute of Fundamental Research, Mumbai 400005, India
Graduate School of Science, Osaka City University, Osaka 558-8585, JapanThe GRAPES-3 is a dense extensive air shower array operating with ∼400 scintillator detectors and it also
contains a 560 m2 tracking muon detector (Eµ >1 GeV), at Ooty in India. 25% of scintillator detectors are
instrumented with two fast photomultiplier tubes (PMTs) for extending the dynamic range to ∼5×103 particles
m−2 . The scintillators, signal processing electronics and data recording systems were fabricated in-house to cut
costs and optimize performance. The muon multiplicity distribution of the EAS is used to probe the compositionof primary cosmic rays below the ‘knee’, with an overlap with direct measurements. Search for multi-TeV γ-rays
from point sources is done with the aid of the muon detector. A good angular resolution of 0.7◦ at 30 TeV, ismeasured from the shadow of the Moon on the isotropic ﬂux of cosmic rays. A sensitive limit on the diﬀuse ﬂux
of 100 TeV γ-rays is placed by using muon detector to ﬁlter the charged cosmic ray background. A tracking
muon detector allows sensitive measurements on coronal mass ejections and solar ﬂares through Forbush decrease
events. We have major expansion plans to enhance the sensitivity of the GRAPES-3 experiment in the areas
listed above.

1. Introduction
The GRAPES-3 (Gamma Ray Astronomy at
PeV EnergieS- phase 3) is a high density extensive air shower (EAS) array designed for a precision study of the cosmic ray (CR) energy spectrum and its nuclear composition using the muon
multiplicity distribution (MMD) in the energy
range from 3×1013eV to 3×1016 eV [1]. The experiment was started with 256 plastic scintillator
detectors (each 1 m2 in area) deployed on a dense
hexagonal pattern with an inter-detector separation of 8 m, at Ooty (2200 m altitude, 11.4◦ N,
76.7◦E) in south India in 2001 as shown in Fig. 1[2]. The array also contains a large area (560 m2 )tracking muon telescope [3] to measure the muoncomponent and obtain the MMD of the EAS. The
muon telescope has also provided new information on solar ﬂares, coronal mass ejections andForbush decrease (Fd) events [4,5].
The occurrence of the ‘knee’ in the CR energy
spectrum around 3×1015 eV is believed to be in∗ deceased

0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.nuclphysbps.2009.09.027

timately linked to the issue of CR origin. But
even after several decades of study, a clear understanding of the origin of the ‘knee’ is yet to
emerge. Data obtained with higher sensitivity
and less uncertainty in the estimate of primaryenergy and composition are expected to provide
a better understanding of this important featureof the high energy astrophysics.
Solar ﬂares accelerate charged particles
through a variety of mechanisms, which may beconstrained through observations at high energies(>10 GeV). We searched for direct emission of
protons of energy 20 GeV in association with an
X17 class solar ﬂare that occurred on 28 October
2003, using a large area tracking muon telescope.
A 99% C.L. upper limit on the ﬂux of protons
due to the solar ﬂare was placed at 1.4×10−6
cm−2 s−1 sr−1 , nearly two orders of magnitude
lower than ﬂux values reported earlier for otherﬂares. New information on the structure andtime evolution of the large Fd observed on 29
October 2003 from nine diﬀerent solid angle bins(∼0.3 sr) showed a remarkably similar behavior,

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S.K. Gupta et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 153–158

Figure 1. The GRAPES-3 array with 257 detectors () and 16 muon detector modules ().

ity and a dynamic range of 1 ms was achieved
which matched the design capability of TDC32.
Subsequently we have developed new 32 channel TDCs using the HPTDC ASIC from CERN,
which allows a time resolution of 100 ps. The
signal processing electronics for the muon detector is relatively simple because of the slower signals from the proportional counters. The entire
data recording electronics for both the muon detector and the air shower array was also developed. This emphasis on in-house development
has the following advantages, namely, (i) a signiﬁcant reduction in cost, (ii) optimization andcustomization of the system according to the experimental requirements, (iii) fast repair in casesof failure, which is very important due to the remote location of Ooty.
2. Experimental details

with an evolution on a time scale of ∼1 h.
An eﬃcient detector system should provide alarge separation between the PMT noise and thesignal from the charged particles in an EAS, andshould also have a uniform spatial response overthe entire area of the scintillator. The original GRAPES-3 trapezoidal shaped detectors, suffered from sizable non-uniformity and delivereda relatively small signal. Therefore, in the newdetectors wave-length shifting (WLS) ﬁbers areused to ensure a large signal, uniform response
and a dynamic range of up to ∼5×103 particles
m−2 . The response of the detector was comparedwith the results of detailed Monte Carlo simulations to obtain a comprehensive understanding of
the entire system [6].
The signal processing involves ampliﬁcationand shaping of the PMT signal into a digitalpulse. The necessary ampliﬁers and discriminator circuits were also developed and fabricatedin-house and an accuracy of 100 ps was achievedin timing shower particles. The arrival time of individual detector signals are measured using time
to digital converters (TDCs) developed in-houseusing the TDC32 ASIC developed by CERN. A
time resolution of 500 ps with excellent linear-In order to achieve the lowest possible energy
threshold, a simple 3-line coincidence of detectors has been used to generate the Level-0 trigger,
which acts as the fast GATE and START for the
analog to digital converters (ADCs) and TDCs,
respectively. As expected, this trigger selects alarge number of very small and local showers andalso larger showers whose cores land very far fromthe area covered by the array. Therefore, it is also
required that at least 10 out of the inner 127 detectors should have triggered their discriminators
within 1 μs of the Level-0 trigger. This Level-1
trigger with an observed EAS rate of 13 Hz is used
to record the charge (ADC) and the arrival time(TDC) of the pulses from each detector [2]. With
the expansion of the array the trigger rate has
increased to 30 Hz. The pulse charge is later converted into the equivalent number of minimumionizing particles (MIPS) using the most probable charge for a single MIP measured. The highsensitivity of scintillator detectors allows the atmospheric radon decay products to be routinely
detected during every episode of rain in Ooty.
Fig. 1 also shows 16 squares in lower left side.
Each square represents a 4-layer tracking muon
telescope module with an energy threshold of
1 GeV for vertical muons. Each layer consistsof 58 proportional counters, each 6 m long with

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155

even-odd, left-right and the shadow of the Moon.
In the ﬁrst method the array is divided into two
sub-arrays of even and odd numbered detectors;
in the second method the concept of left andright half-arrays with a cone-shaped shower front
is used to estimate the energy dependent angular resolution of array. We measured a resolutionof 0.5◦ for the left-right case which is marginallylarger than 0.4◦ for the even-odd method for EASof ≥80 TeV. Reduction in isotropic ﬂux of CRs
due to the shadow of Moon yields a resolution of
0.5◦ at ≥80 TeV [12].
3. GRAPES-3 Results
Figure 2. Observed muon multiplicity as histogram in black for, (a) 104.4 ≤ Ne ≤ 104.6 ,(b) 105.0 ≤ Ne ≤ 105.2 for 5 component based
SIBYLL generator.

10 × 10 cm2 cross-sectional area. The 560 m2
GRAPES-3 muon telescope consists of 4 supermodules, each in turn having 4 modules. The
muon energy threshold of 1 GeV is achieved byplacing concrete of thickness ∼550 g.cm−2 above
the muon telescope.
To improve the angular resolution of GRAPES3, it was necessary to decrease the inter-detector
separation in the array. This approach provides
an excellent compromise between a small area
carpet array and a very large area sparsely instrumented array. However, economic constraints
limit the density of detectors employed in an EAS
array. In the Tibet ASγ [7], CASA [8], EAS-TOP[9], KASCADE [10] and GAMMA [11] arrays, thearea covered by the detectors is 1% of the physical area over which the array is spread. The density of detectors is a key factor that determines
the measurement accuracy of parameters, suchas the shower size ‘Ne ’ and arrival direction. The
GRAPES-3 array is designed to have a dense conﬁguration with a detector coverage of ∼2% of thearray’s total area [2].
The angular resolution of GRAPES-3 has beenmeasured by analyzing EAS data collected in 4
years and by using 3 diﬀerent methods, namely,

In the following we summarize the results obtained from the GRAPES-3 experiment on the
primary composition, diﬀuse γ-ray emission, andon solar ﬂares.
3.1. Primary Composition
We have analyzed 6×108 EAS collected over a
live-time of 5×107s, for shower size Ne , core location (X0 ,Y0 ), age ‘s’ by ﬁtting the NKG function[13] to the lateral distribution. The EAS core is
restricted to 30 m from the centre of array anda condition θ<45◦ is imposed. The MMD is obtained for diﬀerent Ne groups in intervals of 0.2
in log10 (Ne ). A further cut on the distance of the
EAS core, to lie between 60 to 80 m, from thecentre of muon detector is imposed, as shown by
the shaded area in Fig. 1. Monte Carlo simulations of EAS are carried out using the CORSIKA
code [14] for 5 nuclei, namely, H, He, N, Al, and
Fe. These simulations show that an eﬃciency of∼90% is reached at 50 TeV for H and at 100 TeV
for the Fe primaries [2]. The hadronic interactionsin the simulations are described by the SIBYLL[15] and QGSJET [16] generators.
The MMD is also simulated for each of the
5 components using SIBYLL and QGSJET generators for the same range of Ne as the data.
The contribution of each nuclear group is extracted by using a minimization technique described in [17]. The observed MMD is shown in
Fig. 2a, as black histogram for shower size range,
104.4 ≤Ne ≤104.6 . Also shown in Fig. 2a are thecontributions of 5 components, namely, p, He, N,

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S.K. Gupta et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 153–158

1

HEGRA-AIROBICC
Utah-Michigan
EAS-TOP-1

Iγ /ICR Upper Limit

10

CASA
GRAPES-3

-2

10

10-3
10-4
10-5
10

102

3

10
Energy (TeV)

104

Figure 4. Diﬀuse γ-ray ﬂux upper limits

the corresponding data and simulation results forsize range, 105.0 ≤Ne ≤105.2 . The simulations
are used to convert the size into the CR primaryenergy. The 5 component composition extracted
from MMD distributions is shown in Fig. 3 for the
QGSJET (ﬁlled squares) and SIBYLL (ﬁlled circles) generators. These energy spectra are compared with the results from the direct observations also shown in Fig. 3. H and He data aretaken from [18,19,21,22], and CNO, Al, Fe datafrom [19,20,22]. The GRAPES-3 data agree better with the direct measurements when SIBYLL
generator is used, indicating that SIBYLL provides a good description of the hadronic interactions below 1 PeV.

Figure 3. H, He, CNO, Al, Fe spectra from direct
measurements  Ryan [18],  SOKOL [19], 
JACEE [21],  RUNJOB [22], and GRAPES-3• SIBYLL,  QGSJET.

Al, Fe, simulated using SIBYLL. Fig. 2b shows

3.2. Diﬀuse γ-ray limits
The diﬀuse ﬂux of ∼100 TeV γ-rays is produced
from a variety of astrophysical sources, including
the interactions of extremely high energy CRs
with the 2.7◦ K cosmic microwave background(CMB) radiation. These interactions transfer theenergy of CRs into γ-rays of progressively lower
energy that in turn produce e+ e− pairs in the
CMB ﬁeld until the center of mass energy drops
below the pair production threshold which occurs
at ultra-high energies (UHE, Eγ 1014 eV=100
TeV) and then the universe becomes relativelytransparent to the propagation of UHE γ-rays.

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S.K. Gupta et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 153–158

3.3. Solar ﬂare
We had searched for direct emission of protons
of energy 20 GeV in association with an X17
class solar ﬂare on 28 October 2003. The tracking capability of the muon telescope allowed theﬁeld of view to be segmented into 9 bins, (labeled
NE, E, SE, N, V, S, NW, W, SW; N north, E
east, W west, S south and V vertical direction)
each covering a solid angle of ∼0.3 sr. The large
muon rate of 3×106 min−1 permits a high precision study of various solar phenomena. Since
detected >1 GeV muons are secondaries of primary protons of 20 GeV energy, we used theseobservations to place a 99% C.L. upper limit of
1.4×10−6 cm−2 s−1 sr−1 on the wide-angle (∼2.5sr) ﬂux of solar protons. This limit is displayedin Fig. 5 along with results of other groups, including from neutron monitors (NMs) and GOES
data. Using the muon telescope tracking feature
a limit of 4×10−6 cm−2 s−1 sr−1 is placed on the
proton emission along a narrow beam (∼0.3 sr).
New information was obtained on the structure
and time evolution of Fd observed on 29 October2003 by GRAPES-3 [4]. The onset of Fd in 9 solid
angle bins (∼0.3 sr) shows a remarkably similar

10 2
GOES

Integral Flux (cm-2 s -1sr -1)

Thus a pileup of γ-rays at ∼100 TeV is expected.
More exotic scenarios such as the collapse of topological defects created in the early universe could
also result in copious production of 100 TeV γrays.
Several groups in recent times have placed
stringent upper limits on the diﬀuse γ-ray ﬂux
in the several tens of TeV to PeV (1015 eV) energy region which seem to question the validityof certain models of production of the highest energy particles. We have searched for diﬀuse γ-rays
using the GRAPES-3 large area (560 m2 ) muon
telescope (Eμ ≥1 GeV). EAS with cores incident
within the array and unassociated with penetrating tracks in the muon telescope, classiﬁed as μpoor, are considered as candidate γ-ray events.
The μ-poor cut was devised with the aid of simulations of CR and γ initiated EAS. In Fig. 4 thelimits on the ratio Iγ /Ip of the intensities of γ-rays
and CRs, respectively, are shown as a function of
the primary energy. Also shown are the results
reported by other groups [23–26].

1
NM

10

-2

10

-4

10

-6

0.1

1

10

100

1000

Energy (GeV)

Figure 5. Integral protons ﬂux data on 28 Oct
2003; (a) 100-600 MeV GOES-10/11 and subGeV balloon, (b) NM, (c)  20 GeV GRAPES-3
limit, (d)  >10 GeV AGASA 4 Jun 1991, (e) >40 GeV L3 limit 14 Jul 2000, (f)  >500 GeV
Baksan limit on 29 Sep 1989, (g) • NM 15 Apr
2001, (h)  NM 23 Feb 1956.
behavior, with an evolution on a time scale of ∼1
h. The observed muon rates in intervals of 6 minfor the 9 bins are shown in Fig. 6. There is asystematic variation in time, when a speciﬁc feature in muon rate appears in a given solid angle
bin. To highlight this aspect, the data in Fig. 6
are displayed after a grouping in 3 sets labeled“East”, “Center”, and “West”, respectively. The
East set contains data from 3 eastern solid anglebins, namely, NE, E, and SE. Similarly the Centerset contains data from bins N, V, S, and ﬁnally
the West set contains NW, W, SW. For clarity,
data shown in Fig. 6 for the Center and Westsets have been shifted downwards by 4% and 8%,
relative to East set. The 3 triangles in Fig. 6, indicate time of Fd minimum for each set. A powerlaw dependence of magnitude of Fd on the cutoﬀ
rigidity was observed from the muon data over
the range 14.3-24.0 GV, and we precisely measured the spectral slope to be γ=0.53±0.04.
4. Future Plans
We propose to expand the GRAPES-3 arrayto detect CRs up to 1018 eV with the addition

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S.K. Gupta et al. / Nuclear Physics B (Proc. Suppl.) 196 (2009) 153–158% Change in Muon Rate

0

-5

-10

-15

0

4

8
12
16
Time in hours (UTC)20

24

Figure 6. Variation in muon rate on 29 October2003 from 9 solid angle bins grouped into 3 sets;
east (NE, E, SE), center (N, V, S), and west (NW,W, SW). Successive sets shifted downwards by
4% for clarity. Each pointer represents time of
Fd minimum for the 3 sets, respectively.
of new widely spaced detectors. We would also
double the area of the muon detector to increase
sensitivity for detecting γ-rays. The addition of
an imaging Čerenkov telescope to measure the energy of primary CRs is also being contemplated.
Setting up a neutron monitor to supplement information from the muon detector for solar studies is also being considered. Several new groups
from within India have joined the collaborationand some from abroad are in the process of joining us.
Acknowledgment
We thank D.B. Arjunan, G.P. Francis, I.M. Haroon, V. Jeyakumar, K. Manjunath, S. Murugapandian, B. Rajesh, K. Ramadass, C. Ravindran, V. Santosh Kumar and R. Suresh Kumar,
V. Viswanathan for their help in the installationand operation of the experiment.
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